fOUl•rAt, OF GEOPHYSICAL RESEAI•CH, SPACE Pr•¾s•cs VOL. ?4, No. 19, S•P?•MBEI• 1, 1969 Flux and Spectrum of Primary Hydrogen Nuclei near Geomagnetic Equator R. K. PuR• AND P. K. AmTYA Department o• Physics Indian Institute o] Technology, Delhi, India In an emulsion stack exposed at an atmosphericdepth of 3.8 g cm -•' on March 16, 1965, the flux of primary hydrogen nuclei has been estimated equal to (103 _+ 11) particles/m 2 ster sec, correspondingto a threshold rigidity of 16.8 Gr. The regressioncurve for the integral flux of primary hydrogen nuclei above 16.8 Gv versus neutron monitor count rate has been drawn. The ratio of primary hydrogen to helium nuclei measuredsimultaneouslyat the same location yields a value of 72 _+ 1.0. The integral rigidity spectrum of primary hydrogen nuclei can be represented by a spectralindex of --(1.65 _+ 0.12). INTRODUCTION SCANNING ?ROCEDURE The hydrogen nuclei represent both numeri- cally and energetically the mostimportantsingle component of primary radiation. Though exten- sive study of the integral intensity of proton component has been made above a rigidity of 1.3 Gv and 4.5 Gv corresponding to respective locations of Minneapolis and Texas, such observationsat high rigidity of about 17 Gv corresponding to locations aituated near geomagnetic equator have been comparatively fewer. The flux of primary hydrogennuclei has been determinedat Hyderabad (cutoff rigidity • 16.8 Gv) with improvedstatistics during 4he period of solar minimum. The method of Wad- dington [1960] has been followedfor estimating the proton flux from the number of disintegra- tions initiated by relativistic singly charged particles. It has been utilized to draw the re- gression curve by plottingthe flux values against Mr. Washington neutron monitorcount rate and to draw the spectrum of primary hydrogen nuclei. DETAILS OF E•ULSION STAC•C A nuclear emulsion stack, consisting of 28 pelliclesof G5 emulsion,each of size 20 cm X 7 cm X 0.06 cm, has been exposed at Hyder- abad at an average residual atmospheric depth of 3.80 g cm -2for 8.30 hourson March 16, 1965. To avoid ascentcorrection, the stack has been flipped through 90 ø at the ceiling altitude;the 7-cm sidewaskept vertical throughout the time of exposure. A known volume of the emulsion has to be scanned for interactions possibly initiated by protons.As the proton directions are random, the efficiency of picking up starswith zero, one, or two black or grey prongs is very small. Hence a selected volume of this stack has been scannedunder a total magnification of X225 for stars induced by relativistic singly charged particles having at least three black or grey prongs, i.e. n• _• 3 (where n• is equal to the sum of black and grey prongs).The corrections for stars with n• • 3 has been made later. The interactions, so observed,have been carefully examinedunder a high magnification for any associated shower tracks in the upper hemis- phere. The projected zenith angle, 0•, of such a relativistic particle inducing a star has been measured with respect to the vertical direc- tion. Only those interactionsinduced by mini- mum ionizing tracks that have a zenith angle •60 ø have been selected. In an effective vol- ume of 20.52 cm •, • total of 434 stars satisfying this selection criteria have been found. SCANNING EFFICIENCY About 50% of the total volume has been re- scannedby another worker, and an over-all scanningefficiencyhas been estimated to be nearly 100%. A comparison has been made for na distribu- tion of the stars observed in the presentwork with that'obtained by Daniel et, al. [1960]and Winzeler et at. [1960] from the disintegrations 4787